Pyrotechnic Method for the Stabilisation of Low Bearing Capacity Subsoil

ABSTRACT

A pyrotechnic method for stabilisation of low bearing capacity subsoil, that consists of determination of the parameters of the layer of soil to be stabilised, drilling nontabulated, vertical shot-holes at the intersection points of a square or equilateral triangle grid, selection, preparation and firing the oblong charges, characterised by the fact that the shot-holes with a diameter from 0.02 to 1.5 m and a depth of up to 50 m, are drilled with a water flush or a mud or clay suspension, at a spacing of 3 to 25 m, this spacing distance being inversely proportional to the thickness of the layer of soil that is to be stabilised. The charges, the total length of which cannot exceed the thickness of the layer to be consolidated, are composed of several small “sub-charges”, spaced at distances from 1 to 10 m down the shot hole. The bottom end of each charge is blocked with a rod the diameter of which is greater than that of the shot hole. The sub-charges are fired successively from the top one to the bottom one, with a delay of between 0.05 to 10 milliseconds. The sequence of firing starts at one end of the area of soil to be consolidated. The charges are fired individually or in small groups at time intervals. Before the drilling of the shot holes a layer of loose soil, the thickness of which should vary from 0.5 to 20 m, is deposited on the area to be stabilized. After the explosions the non-cohesive soil fills the shot holes. The “sub-charges” are vertically fastened in the shot holes with wood, plastic or metal distance making rods.

The subject of the invention is a pyrotechnic method for the stabilisation of low bearing capacity subsoil, also called weak soil. This method is used in civil and harbour engineering, and during the construction of motorways, roads, airports, railway lines etc.

Pyrotechnic methods use the energy of shock waves resulting from the explosion of charges. The method for the consolidation of saturated non-cohesive soil, set out in Polish patent description no. 151405, consists of the use of charges with a length of at least a quarter of the thickness of the soil layer to be consolidated, and with a mass g=k×H^(1/2) kg/running metre, where H is the thickness of the consolidated soil layer and k is an empirical factor which depends on the particle size distribution and compaction of the soil, and is selected from the range of 0.3 to 1.0 kg/m². Charges are placed in non-tabulated shot holes, in at least three parallel layers. In each layer the charges are placed at the vertexes of a grid of equilateral triangles. The length of the sides of these triangles is a quarter of the radius of effective action of the shock wave. The layers are arranged such that the vertexes of the triangles of one layer are shifted in relation to the vertexes of the triangles of the next layer, by half the length of a triangle side. The distance between layers is such that the ends of the charges of one layer are on the plane which bisects lengthways the charges of the next layer. The shot holes, the diameters of which vary from 15 to 30 cm, are drilled with a bentonite suspension. The use of charges with a diameter of 5 to 6 cm is recommended.

Another method for the consolidation of cohesive soil lying under a layer of saturated non-cohesive soil, is set out in the Polish patent description no. 145208. This involves the use of explosive charges fired in three series. The first series of oblong charges are placed in the cohesive soil in the shot holes, which should reach down to load bearing soil. The charges are arranged at an equal distance of four times the radius of the effective impact of the explosion of the charges, which distance depends on the weight of the explosive in the charges. For the subsequent series of explosions the arrangement of the charges is different.

For the technological process of construction works it is very important that soil consolidation immediately follows subsoil stabilization works. In that way, soil settlement during the construction works prevents later settlement resulting from service loads that could exceed permissible limits.

The invention proposes a pyrotechnic method of soil stabilisation characterised by shot holes with a diameter from 0.02 to 1.5 m and a depth of up to 50 m, drilled with a water flush or a mud or clay suspension, at a spacing of 3 to 25 m, this spacing distance being inversely proportional to the thickness of the layer of soil that is to be stabilised. The charges, the total length of which cannot exceed the thickness of the layer to be consolidated, are composed of several small “sub-charges”, spaced at distances from 1 to 10 m down the shot hole. The bottom end of each charge is blocked with a rod the diameter of which is greater than that of the shot hole. The sub-charges are fired successively from the top one to the bottom one, with a delay of between 0.05 to 10 milliseconds. The sequence of firing starts at one end of the area of soil to be consolidated. The charges are fired individually or in small groups at time intervals.

As a result of the firing of the charges, soil stabilisation progresses linearly across the area to be stabilised. After each explosion the vertical shot holes are filled with loose ground gravitationally, manually, or mechanically so as to make piles. During the firing of neighbouring charges these piles function as vertical drains that allow water to flow out. The impact of the subsequent firings consolidates the saturated soil in the piles.

For cohesive, non-loose ground, such as peaty ground, it is recommended that before the drilling of the shot holes, a layer of loose soil, or better, sand, is deposited on the area to be stabilised. The thickness of this layer should vary from 0.5 to 20 m. In particular geodesic conditions the thicker layer of 20 m is favourable. In, for instance, the case of stabilisation works made from an embankment like a silted up pier. In the case of harbour engineering, the thickness of silt often exceeds 20 m. After the explosions the non-cohesive sand or soil fills the shot holes which then function as vertical drains.

It is also recommended, that the sub-charges are vertically fastened with distance making wood, plastic or metal rods.

The invented method presented above produces compacted subsoil with a structure that enables rapid soil consolidation, the degree of consolidation being S_(k)>0.9 which guarantees that the service settlement will be lower than the maximum acceptable, as determined by building standards.

The following two descriptions of subsoil stabilisation projects will help to fully explain this invention.

EXAMPLE 1

During the construction of the approaches to a new river bridge, the stabilisation of subsoil, composed of peat and sediments, with a plasticity degree L between 0.60 and 0.80, was carried out. The thickness of the subsoil to be stabilised varied from 6 to 16 m, with the roof of the weak soil layer from 1 m below ground level and the floor up to 16 m below that. Before stabilisation the area was covered with a layer of poured sand 1 m thick, which facilitated the movement of machines and guaranteed spontaneous filling of shot holes. This limited the need for dozers or manual filling. The shot holes, with diameters varying from 0.05 to 0.25 m, right down to the floor of the layer to be stabilised, were drilled by a self-propelled drilling machine. Drillings were flushed with water, without a drilling fluid. The spacing of the shot holes, drilled at the intersection points of a square grid, depended on the depth of the hole: where thickness of the soil layer to be stabilised was from 13 to 16 m−the square side R equalled 4 m. Where it was from 9 to 13 m−R=6 m. From 6 to 9 m−R=6 m. The diameter of the sub-charges varied from 2.5 to 10 cm, and the distance between them, determined by wooden distance rods, was less than 2 m. The total weight of dynamite or dynammonite in a charge did not exceed 4.0 kg. The firing was done in one series of explosions. The sub-charges were fired successively from the top one to the bottom one, with a delay of from 0.05 to 5 milliseconds. The bottom end of each charge was blocked with a wooden rod with a diameter greater than that of the shot hole, thus containing the explosion in the shot hole. The charges were fired one after another, beginning from one end of the stabilised subsoil area. Stabilisation of the subsoil layer was the result of superposition of separate stabilisations achieved at a distance of 0.75×R between the charges. Four and a half months after the stabilisation works, soil consolidation of degree S_(k)=0.92 was obtained.

EXAMPLE 2

During the construction of a motorway the stabilisation of subsoil composed of weak dusty clay, sediments, and weak, fine grained-sand with a degree of plasticity I_(L) from 0.55 to 0.85 and a degree of compaction I_(D) from 0.25 to 0.40 was carried out. The thickness of the subsoil layer to be stabilised varied from 5 to 6 m. The roof of the weak soil layer was from 1 m to 1.5 m below ground level and the floor of the weak soil layer about 5 to 6 m deeper. The diameter of the charges was from 1.5 cm to 5 cm, and their length equalled the thickness of the stabilised subsoil. The shot holes were drilled with steel boring rods. The total weight of dynamite, dynammonite or ammonite in a charge did not exceed 2.5 kg. Each charge was fired in such a way that the upper sub-charge had a fuse with a delay of 1 millisecond, the middle one had a fuse with a delay of 3 milliseconds, and the bottom one a fuse with a delay of 5 milliseconds. The charges were spaced R=5 m, on a grid of equilateral triangles. It was assumed that that the stabilised subsoil area equalled 0.6 R from the axis of the charge. The charges were fired one by one. Twenty-one days after the subsoil stabilisation was carried out the average settlement of the ground was 0.27 cm. Later measurements proved the effectiveness of the method—the subsoil settlement was lower than that allowed by building standards. 

1-3. (canceled)
 4. A method of stabilizing soil, comprising the steps of: drilling a plurality of shot holes at intersection points of one of a square grid and an equilateral triangle grid; inserting a charge in each shot hole, the charge comprising a plurality of sub-charges, a total length of the charge not exceeding a thickness of a layer of soil to be consolidated; blocking a bottom end of the charge with a rod having a diameter greater than a diameter of the shot hole; and firing charges successively from one end of an area of soil to be consolidated and at time intervals, the sub-charges of each charge being fired successively from a top sub-charge to a bottom sub-charge.
 5. The method of claim 4, wherein the plurality of shot holes are drilled with at least one of a water flush, a mud suspension and a clay suspension.
 6. The method of claim 4, wherein the shot holes are drilled at a spacing distance of about 3 meters to about 25 meters.
 7. The method of claim 6, wherein the spacing distance between shot holes is inversely proportional to the thickness of the layer of soil to be stabilized.
 8. The method of claim 4, wherein stabilization progresses linearly across the area of soil to be stabilized.
 9. The method of claim 4 further comprising the step of filling the shot holes with loose ground after explosion.
 10. The method of claim 9, wherein the step of filling the shot holes with loose ground is at least one of gravitational filling, mechanical filling and manual filling.
 11. The method of claim 4, wherein charges are fired individually.
 12. The method of claim 4, wherein charges are fired in groups.
 13. The method of claim 4, wherein each shot hole has a diameter of about 0.02 meters to about 1.5 meters and a depth of up to about 50 meters.
 14. The method of claim 4, wherein sub-charges are spaced from about 1 meter to about 10 meters down the shot hole.
 15. The method of claim 4, wherein sub-charges are fired with a delay of between about 0.05 milliseconds to about 10 milliseconds.
 16. A pyrotechnic method for stabilization of soil, comprising the steps of: drilling vertical shot holes at intersection points of one of a square grid and an equilateral triangle grid, the shot holes being spaced from about 3 meters to about 25 meters and each shot hole having a diameter of about 0.02 meters to about 1.5 meters and a depth of up to about 50 meters; inserting a charge in each shot hole, each charge comprising a plurality of sub-charges spaced from about 1 meter to about 10 meters down the shot hole, a total length of each charge not exceeding a thickness of a layer of soil to be stabilized; a bottom end of each charge blocked with a rod having a diameter greater than the diameter of the shot hole; and firing charges successively from one end of an area of soil to be consolidated and at time intervals and firing sub-charges successively from a top sub-charge to a bottom sub-charge with a delay of between 0.05 milliseconds to 10 milliseconds.
 17. The method of claim 16 further comprising the step of depositing on the area of soil to be stabilized a layer of loose soil prior to drilling the shot holes.
 18. The method of claim 16, wherein the layer of loose soil is a layer of sand.
 19. The method of claim 16 further comprising the step of filling the shot holes with loose ground after explosion to form piles, the piles forming vertical drains.
 20. A pyrotechnic method for stabilization of low bearing capacity subsoil, comprising the steps of: determining parameters of a layer of soil to be stabilized; drilling vertical shot holes at intersection points of one of a square grid and an equilateral triangle grid, shot holes spaced from about 3 meters to about 25 meters, each shot hole having a diameter of about 0.02 meters to about 1.5 meters and a depth of up to about 50 meters; inserting a charge in each shot hole, each charge comprising a plurality of sub-charges spaced from about 1 meter to about 10 meters down the shot hole, a total length of each charge not exceeding a thickness of the layer of soil to be stabilized; blocking a bottom end of each charge with a rod having a diameter greater than the diameter of the shot hole; and firing charges successively from one end of an area of soil to be consolidated and at time intervals and firing sub-charges successively from a top sub-charge to a bottom sub-charge with a delay of between 0.05 milliseconds to 10 milliseconds.
 21. The method of claim 20 further comprising the step of depositing on the area of soil to be stabilized a layer of loose soil prior to drilling the shot holes.
 22. The method of claim 21, wherein a thickness of the layer of loose soil is between about 0.5 meters to about 20 meters.
 23. The method of claim 20, wherein the sub-charges are fastened vertically in each shot hole with distance making rods. 